1. Field of the Invention
The present invention relates to the particles for display media and the information display panel using the above particles, in which the display media are sealed between two substrates, at least one substrate being transparent, and, in which the display media, to which an electrostatic field is applied, are made to move so as to display information such as an image.
2. Description of Related Art
As an information display device substitutable for liquid crystal display (LCD), information display devices with the use of technology such as an electrophoresis method, an electro-chromic method, a thermal method, dichroic-particles-rotary method are proposed.
As for these information display devices, it is conceivable, as compared with LCD, as inexpensive visual display device of the next generation from a merit having wide field of vision close to normal printed matter, having smaller consumption, or having a memory function, and spreading out to a display for portable device and an electronic paper is expected. Recently, electrophoresis method is proposed that microencapsulate dispersion liquid made up with dispersion particles and coloration solution and dispose the liquid between faced substrates, and also it is expected.
However, in the electrophoresis method, there is a problem that a response rate is slow by the reason of viscosity resistance because the particles migrate among the electrophoresis solution. Further, there is a problem of lacking imaging repetition stability, because particles with high specific gravity of titanium oxide is scattered within solution of low specific gravity, it is easy to subside, difficult to maintain a stability of dispersion state. Even in the case of microencapsulating, cell size is diminished to a microcapsule level in order to make it hard to appear, however, an essential problem was not overcome at all.
Besides the electrophoresis method using behavior in the solution, recently, a method wherein electro-conductive particles and a charge transport layer are installed in a part of the substrate without using solution is proposed. [The Imaging Society of Japan “Japan Hardcopy '99” (Jul. 21-23, 1999) Transaction Pages 249-252] However, the structure becomes complicated because the charge transport layer and further a charge generation layer are to be arranged. In addition, it is difficult to constantly dissipate charges from the electro-conductive particles, and thus there is a drawback on the lack of stability during display rewriting.
As one method for overcoming the various problems mentioned above, an information display panel is known, in which the display media are sealed between two substrates, at least one substrate being transparent, and, in which the display media, to which an electrostatic field is applied, are made to move so as to display information such as an image. In the information display panel, in which the display media are moved by an electrostatic field so as to display information such as an image, as for the particles constituting the display media, the inventors develop particles for display media utilizing styrene resin as particle raw materials, and the particles for display media mentioned above are used for the image display panel mentioned above.
In the particles for display media utilizing the styrene resin for the particle raw materials as mentioned above, since a glass transition temperature Tg of the resin constituting the particles for display media is about 90° C. and thus a heat resistance is low, it is difficult to use them under a high temperature. Therefore, it is necessary that the resin constituting the particles for display media has a high heat resistance. Moreover, since the particles for display media constituting the display media are generally formed by kneading the raw materials and crushing them, it is necessary that the resin constituting particles for display media has a thermoplasticity. Further, in the case such that the resin that is difficult to crush is used as the resin constituting the particles for display media, since a pressure during the crushing operation becomes high and an amount of a pigment in the particles is decreased due to a dropout of titanium oxide and so on used as the pigment, a degree of whiteness is deteriorated especially in the case of using white pigments. Therefore, it is necessary that the resin constituting the particles for white color display media is easy to crush (that is, the resin has a low impact strength).
According to the requirements mentioned above, whether or not general thermoplastic resins (acrylic resin, styrene resin, polycarbonate rein, polypropylene resin, polyethylene terephthalate resin) are used as the resin constituting the particles for display media is investigated. As a result, it is understood that the resin having a high heat resistance is difficult to crush since it has a high strength, and the resin having a low impact strength has a low glass transition temperature Tg and a low heat resistance. Therefore, it is difficult to find the resin that satisfied all the requirements mentioned above. However, the inventors are confirmed that, in the case of using cycloolefin resin, there are cases that satisfy all the requirements mentioned above.